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><H1
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><A
NAME="SEC-EVENTS"
>Events</A
></H1
><P
>In addition to the signal mechanism described above, there is a set
of <I
CLASS="EMPHASIS"
>events</I
> that reflect the X event mechanism. Callbacks may
also be attached to these events. These events are:</P
><P
></P
><UL
COMPACT="COMPACT"
><LI
><SPAN
> event</SPAN
></LI
><LI
><SPAN
> button_press_event</SPAN
></LI
><LI
><SPAN
> button_release_event</SPAN
></LI
><LI
><SPAN
> scroll_event</SPAN
></LI
><LI
><SPAN
> motion_notify_event</SPAN
></LI
><LI
><SPAN
> delete_event</SPAN
></LI
><LI
><SPAN
> destroy_event</SPAN
></LI
><LI
><SPAN
> expose_event</SPAN
></LI
><LI
><SPAN
> key_press_event</SPAN
></LI
><LI
><SPAN
> key_release_event</SPAN
></LI
><LI
><SPAN
> enter_notify_event</SPAN
></LI
><LI
><SPAN
> leave_notify_event</SPAN
></LI
><LI
><SPAN
> configure_event</SPAN
></LI
><LI
><SPAN
> focus_in_event</SPAN
></LI
><LI
><SPAN
> focus_out_event</SPAN
></LI
><LI
><SPAN
> map_event</SPAN
></LI
><LI
><SPAN
> unmap_event</SPAN
></LI
><LI
><SPAN
> property_notify_event</SPAN
></LI
><LI
><SPAN
> selection_clear_event</SPAN
></LI
><LI
><SPAN
> selection_request_event</SPAN
></LI
><LI
><SPAN
> selection_notify_event</SPAN
></LI
><LI
><SPAN
> proximity_in_event</SPAN
></LI
><LI
><SPAN
> proximity_out_event</SPAN
></LI
><LI
><SPAN
> visibility_notify_event</SPAN
></LI
><LI
><SPAN
> client_event</SPAN
></LI
><LI
><SPAN
> no_expose_event</SPAN
></LI
><LI
><SPAN
> window_state_event</SPAN
></LI
></UL
><P
>In order to connect a callback function to one of these events you
use the function g_signal_connect(), as described above, using one of
the above event names as the <TT
CLASS="LITERAL"
>name</TT
> parameter. The callback
function for events has a slightly different form than that for
signals:</P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>gint callback_func( GtkWidget *widget,
                    GdkEvent  *event,
                    gpointer   callback_data );</PRE
></TD
></TR
></TABLE
><P
>GdkEvent is a C <TT
CLASS="LITERAL"
>union</TT
> structure whose type will depend upon 
which of the above events has occurred. In order for us to tell which event
has been issued each of the possible alternatives has a <TT
CLASS="LITERAL"
>type</TT
>
member that reflects the event being issued. The other components
of the event structure will depend upon the type of the
event. Possible values for the type are:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>  GDK_NOTHING
  GDK_DELETE
  GDK_DESTROY
  GDK_EXPOSE
  GDK_MOTION_NOTIFY
  GDK_BUTTON_PRESS
  GDK_2BUTTON_PRESS
  GDK_3BUTTON_PRESS
  GDK_BUTTON_RELEASE
  GDK_KEY_PRESS
  GDK_KEY_RELEASE
  GDK_ENTER_NOTIFY
  GDK_LEAVE_NOTIFY
  GDK_FOCUS_CHANGE
  GDK_CONFIGURE
  GDK_MAP
  GDK_UNMAP
  GDK_PROPERTY_NOTIFY
  GDK_SELECTION_CLEAR
  GDK_SELECTION_REQUEST
  GDK_SELECTION_NOTIFY
  GDK_PROXIMITY_IN
  GDK_PROXIMITY_OUT
  GDK_DRAG_ENTER
  GDK_DRAG_LEAVE
  GDK_DRAG_MOTION
  GDK_DRAG_STATUS
  GDK_DROP_START
  GDK_DROP_FINISHED
  GDK_CLIENT_EVENT
  GDK_VISIBILITY_NOTIFY
  GDK_NO_EXPOSE
  GDK_SCROLL
  GDK_WINDOW_STATE
  GDK_SETTING</PRE
></TD
></TR
></TABLE
><P
>So, to connect a callback function to one of these events we would use
something like:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>g_signal_connect (G_OBJECT (button), "button_press_event",
                  G_CALLBACK (button_press_callback), NULL);</PRE
></TD
></TR
></TABLE
><P
>This assumes that <TT
CLASS="LITERAL"
>button</TT
> is a Button widget. Now, when the
mouse is over the button and a mouse button is pressed, the function
button_press_callback() will be called. This function may be declared as:</P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>static gboolean button_press_callback( GtkWidget      *widget, 
                                       GdkEventButton *event,
                                       gpointer        data );</PRE
></TD
></TR
></TABLE
><P
>Note that we can declare the second argument as type
<TT
CLASS="LITERAL"
>GdkEventButton</TT
> as we know what type of event will occur for this
function to be called.</P
><P
>The value returned from this function indicates whether the event
should be propagated further by the GTK event handling
mechanism. Returning TRUE indicates that the event has been handled,
and that it should not propagate further. Returning FALSE continues
the normal event handling.  See the section on
<A
HREF="c1799.html"
>Advanced Event and Signal Handling</A
> 
for more details on this propagation process.</P
><P
>For details on the GdkEvent data types, see the appendix entitled
<A
HREF="a2769.html"
>GDK Event Types</A
>.</P
><P
>The GDK selection and drag-and-drop APIs also emit a number of events which
are reflected in GTK by the signals. See <A
HREF="x1941.html#SEC-SIGNALSONSOURCEWIDGETS"
>Signals on the source widget</A
> and <A
HREF="x1941.html#SEC-SIGNALSONDESTWIDGETS"
>Signals on the destination widget</A
>
for details on the signatures of the callback functions for these signals:</P
><P
></P
><UL
COMPACT="COMPACT"
><LI
><SPAN
> selection_received</SPAN
></LI
><LI
><SPAN
> selection_get</SPAN
></LI
><LI
><SPAN
> drag_begin_event</SPAN
></LI
><LI
><SPAN
> drag_end_event</SPAN
></LI
><LI
><SPAN
> drag_data_delete</SPAN
></LI
><LI
><SPAN
> drag_motion</SPAN
></LI
><LI
><SPAN
> drag_drop</SPAN
></LI
><LI
><SPAN
> drag_data_get</SPAN
></LI
><LI
><SPAN
> drag_data_received</SPAN
></LI
></UL
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